Morphology-Patterned Anisotropic Wetting Surface for Fluid Control

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Morphology Patterned Anisotropic Wetting Surface for Fluid Control and Gas-Liquid Separation in Microfluidics Shuli Wang, Nianzuo Yu, Tieqiang Wang, Peng Ge, Shunsheng Ye, Peihong Xue, Wendong Liu, Huaizhong Shen, Junhu Zhang, and Bai Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01785 • Publication Date (Web): 29 Apr 2016 Downloaded from http://pubs.acs.org on May 7, 2016

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ACS Applied Materials & Interfaces

Morphology Patterned Anisotropic Wetting Surface for Fluid Control and Gas-Liquid Separation in Microfluidics Shuli Wang,† Nianzuo Yu,† Tieqiang Wang,‡ Peng Ge,† Shunsheng Ye,† Peihong Xue,† Wendong Liu,† Huaizhong Shen,†Junhu Zhang*,†, and Bai Yang† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Changchun, Jilin University, 130012, P. R. China E-mail: [email protected] ‡ Research Center for Molecular Science and Engineering, Northeastern University, Shenyang, 110004, P. R. China Keywords: Microfluidics, Surface patterning, Anisotropic wetting, Fluid control, Gas-liquid separation Abstract: This article shows morphology patterned stripes as a new platform for directing flow guidance of fluid in microfluidic devices. Anisotropic (even unidirectional) spreading behavior due to the anisotropic wetting of underlying surface is observed after integrating morphology patterned stripes with Y-shaped microchannel. The anisotropic wetting flow of fluid is influenced by the applied pressure, dimension of the patterns, including period and depth of the structure, and size of the channels. Fluids with different surface tension show different flowing anisotropy in our micro-device. Moreover, the morphology patterned surfaces could be used as a micro-valve and gas-water separation in the microchannel was realized using the unidirectional flow of water. Therefore, benefiting from their good performance and simple fabrication process, morphology patterned surfaces are good candidates to be applied in controlling fluid behavior in microfluidics. 1. Introduction In recent years, microfluidics has shown broad applications in chemistry, biochemistry and medical detection due to their feature advantages such as reduced sample volume, low cost, portability and shortened analysis time.1-6 In a microchannel fluidic chip, guidance and operation of the fluid play important roles in rapid and precise analytical functions.7-8 In passive microchannels, surface modification can apparently improve fluidic flow characters or 1

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even realize some smart functions by surface-directed flow guidance.9-11 So far, several surface modification approaches have been demonstrated to guide fluid motion in microchannels, such as self-assembling monolayer (SAM) chemistry, multilayer stream laminar flow, photolithographic method, and plasma treatment etc.9-10,12-16 Modification of microfluidic channels not only can guide the flow path of the fluids, but also improve various analytical functions, including mixing, chemical reaction, extraction, and separation. In addition to these approaches, directional wetting of asymmetric structures has become a promising alternative to control liquid flow in recent years.17-19 For example, Kim et al. reported controlling fluidic motion via unidirectional wetting on stooped nanohairs and these structures were useful for liquid handling in microfluidic devices or flow valves.17 Bae et al. realized in situ fabrication of asymmetric ratchet structures within fluidic channels, and this structure could control the fluid speed in predefined regions and could be used as on-chip timers in split microchannels.20 Zhang et al. reported the utilization of Janus Si elliptical pillar arrays with anisotropic wettability to control liquid flow in a T-shaped microchannel. Due to the anisotropic wetting of the Janus Si arrays, the aqueous solution was directed to the hydrophilic modified direction, while flow spreading in another outlet was suppressed.21 In these microchannels, the inhomogeneous distribution of liquid is dominated by the anisotropic surfaces, which direct liquid to flow in a particular direction. Thanks to the extensive research on asymmetric structures inspired from nature world, a variety of anisotropic wetting and directional patterned surfaces are prepared and investigated.22-28 Among the anisotropic wetting structures, stripes were one of the most attractive structures due to their relatively simple fabrication approaches and excellent anisotropic wetting properties.29-33 Previously, we reported the integration of chemically patterned surfaces of periodic hydrophilic-hydrophobic stripes with Y-shaped channel and found the fluid showed anisotropic flow characters in the channel.34 In this work, we report morphology patterned surfaces of silicon stripe structure for fluid controlling in microchannel. 2


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Fluids exhibit anisotropic (even unidirectional) flow in channels upon morphology patterned stripes due to the anisotropic wettability of the surfaces. The anisotropic wetting flow of fluid is influenced by the applied pressure, period of the stripe patterns and depth of the stripe structure. Most important of all, the gas-water interface formed inside outlet channel in perpendicular direction is stable, which make such micro-device possible for practical applications. The flowing distances of fluids with different surface tension are different inside outlet channel in perpendicular direction. In addition, considering the anisotropic flow of fluid, the morphology patterned surface could be used as a micro-valve. Finally, benefiting from stable gas-water interface, we realized gas-water separation based on the unidirectional flow of water, which enriches applications of patterned surfaces in microfluidics. 2. Experimental Section 2.1 Materials Silicon substrates were cleaned by immersion in piranha solution (7:3 concentrated H2SO4/30%H2O2) for 5 h at 70 oC to create a hydrophilic surface and rinsed repeatedly with Milli-Q water (18.2 MΩ cm-1) and ethanol. The substrates were dried in nitrogen gas before use. Photoresist (BP212-37 positive photoresist, Kempur Microelectronics, Inc., Beijing) was diluted with 1-methoxy-2-propanol acetate (MPA) before use. SG-2506 borosilicate glass (with 145 nm thick chrome film and 570 nm thick positive S-1805 type photoresist, Changsha Shaoguang Chrome Blank Co. Ltd.) was applied as the initial wafer for mold fabrication. Sylgard 184 elastomer base and curing agent for polydimethylsiloxane (PDMS) were purchased from Dow Corning (Midland, MI). Trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFS) and 3-Triethoxysilylpropylamine (APTES)，HAuCl4·3H2O (99%) and trisodium citrate (99%) were purchased from Aldrich. Sulfuric acid, hydrogen peroxide, sodium hydroxide and ethanol were used as received. 2.2 Fabrication of Photoresist stripes and hydrophobic Silicon stripes

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Patterned surfaces with photoresist stripes were fabricated by conventional photolithography.35 Reactive ion etching (RIE) method was used to produce Si slides with stripe structure, and the prefabricated photoresist stripes were used as masks. RIE of silicon was performed using Plasmalab 80 Plus (Oxford Instrument) with a gas mixture of CHF3 at 30 sccm and SF6 at 4 sccm. Total gas pressure was 5 mTorr; the RF power and the ICP power were 20 and 100 W, respectively. After etching, the substrates were rinsed in ethanol to remove the residual photoresist and dried in nitrogen gas. Finally, oxygen plasma treatment and chemical vapor deposition of PFS were performed to provide the Si stripes with hydrophobic chemistry. The surface morphology of the photoresist stripes and Si stripes were characterized by Olympus fluorescence microscope (BX51) and scanning electron microscopy (SEM, JEOL FESEM 6700F electron microscope), and contact angle measurements (Dataphysics OCA20) were performed to study the wetting properties of water on these surfaces. 2.3 Fabrication of Si stripes with increased roughness To fabricate Si stripes with different roughness, Si stripes further etched. In the RIE process, we used Au nanoparticles (NPs) as etching masks. First, Au NPs were synthesized using a general seeded growth route.36 The Au NPs synthesized by this method have negative charges, and they can absorb to surfaces with positive charges by electrostatic interaction.37 Before absorption of Au NPs, Si stripes are treated by oxygen plasma and absorbed one layer of PDDA. Then, the Au NPs absorbed to the Si stripes covered by one layer of PDDA. After 3 hours of absorption, the surfaces were rinsed by diluted water. Then, RIE process was performed to the Au NPs absorbed Si stripes. Before etching Si, the exposed PDDA layer was removed by oxygen plasma for 2 min (RF = 60W, ICP = 0 W, total pressure = 10 mTorr). After RIE of silicon for predefined duration, the Au NPs masks were removed by gold etchant. The morphology of the nanoscale structures were characterized by atomic force microscopy

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(AFM), and surface roughness was measured using the AFM software. Finally, the Si stripes with different roughness were modified with PFS monolayer. 2.4 The Flow Behavior of Water in the Microchannel upon the Morphology Patterned Surfaces Fabrication of glass mold and PDMS microchannels were prepared as reported earlier.38 After detached from the glass mold, the PDMS channel was compressed onto the morphology patterned surfaces, and connected to a Microfluidic Flow Control System (MFCS and FLOWELL, FLUIGENT) using a polytetrafluoroethylene (PTFE) pipe. The fluids in the channels were pressure-driven and the applied pressure was controlled by the MFCS. The flow behavior of the fluids was recorded with an Olympus fluorescence microscope. The flow speed of water is acquired according to the recording video, and flow rate is calculated on the basis of the flow speed and size of the microchannels. 3. Results and Discussions 3.1 Anisotropic Wetting Property of the Morphology Patterned Surfaces The photoresist stripe patterns are prepared by photolithographic method, and Si stripes are obtained by RIE of Si slides with photoresist stripes as masks. Figure 1a is the microscopy image of the as-prepared photoresist stripes. The stripes are 10 µm in width, and the pitches between the stripes are also 10 µm. Figure 1b is the SEM image of the Si stripes, it shows that the stripe structures are about 600 nm in depth while the top and bottom surfaces of the structure are flat. Next, the Si stripes were modified with hydrophobic PFS monolayer through chemical vapour deposition method. To study the wetting property of the prepared morphology patterned surfaces, we measured the water contact angles (WCA) on the patterned surfaces and found water exhibited strong anisotropic wettability on them. As shown in Figure 1c and 1d, the WCA viewing from the parallel direction (141.0o) was larger than that viewing from perpendicular direction (121.2o) on hydrophobic Si stripes. This

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anisotropic wettability is caused by the different energy barrier level between the direction along and perpendicular to the stripes.33 3.2 Unidirectional Flow of Fluid Recently, microfluidic channels that use capillary wicking or directional wetting to control liquid flow have become promising alternatives to conventional systems that require external pumps. In an earlier report, we demonstrated chemically patterned anisotropic wetting surfaces could control the flow behavior of water in microchannels.34 To explore the potential applications of morphology patterned surfaces in microfluidics, a Y-shaped microchannel was compressed upon the patterned surfaces (Figure 2a) and flow behavior of fluid in the microchannel was investigated. Figure 2b shows the optical microscopy image of the Y-shaped microchannel. In our microfluidic chip, outlet 1 is parallel to the stripes direction while outlet 2 is perpendicular. Water was injected into an inlet channel using a PTFE pipe and the outlet channels were both connected to atmosphere. First, we studied the flow behavior of water in microchannel on top of Si stripes. As shown in Figure 2c, when the applied pressure is 45 mbar, water flows into the outlet 1 while stops at the beginning of outlet 2. It shows unidirectional flow behavior in the microchannel (Supporting Information, Video S1), and the fluid motion into outlet 2 channel is largely suppressed. This should be ascribed to energy barrier at the beginning of outlet 2, because in the perpendicular direction, the Si stripes and PDMS channel walls are all hydrophobic and water needs to overcome larger energy barrier when compared to parallel direction.30,33 Thus, the Si stripes can guide the flow behavior of water in microchannels. In contrast, we studied the flow motion of water in microchannel upon PFS modified Si surfaces without stripe patterns, and no difference was found in two outlets (Figure 2d), suggesting surfaces without stripe patterns could not direct the flow behavior due to their isotropic wetting property. Thus, anisotropic wetting of the morphology patterned surfaces induces the unidirectional flow character of water in microchannel. Apart from this, we found that water also shows same unidirectional flow 6


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characteristic in microchannel upon photoresist stripes (Figure 2e), which further verifying the controlling capacity of the morphology patterned surfaces in the microchannel. But the physicochemical stability of the photoresist is poor compared to Si stripes, so we use patterned surfaces of Si stripes in the following experiments. We analyzed the recording videos in detail to reveal the nature of unidirectional flow of water in microchannels. The detailed flowing process consists of two stages: First, when water arrives at the Y-junction, it flows forward into outlet 1 step by step until it fills up the whole section of the microchannel. At this point, a gas-water interface formed at the beginning of the outlet 2. In the second stage, water flows through outlet 1, while in outlet 2 the position of gas-water interface is unchanged and water stops at the beginning (details seen in Figure S1). The water motion in outlet 1 is same but in outlet 2 is different compared with that in microchannel built upon chemically patterned surface.34 Compared to chemically patterned surface, the gas-water interface in outlet 2 upon morphology patterns is more stable, even when water flows through outlet 1 and accumulates to a big droplet, the position of gas-water interface remains unchanged. For chemically patterned surface, the interface is unstable and changes over time. This is the reason why water presents unidirectional flow in microchannel upon morphology patterned surface, while anisotropic flow on chemically patterned surface.The stability of the gas-water interface is quite different between the two kinds of patterned surfaces and importance of it will be discussed later. The unidirectional or anisotropic flow of water in microchannels on both physically and chemically patterned surface is originated from the existence of energy barrier in the perpendicular direction, the mechanism of the occurance of energy barrier on both kinds of surface is shown in Figure S2. In our micro-device, fluids are pressure-driven, so value of driving pressure may affect the flowing character of fluid in the microchannel. Therefore, flow behavior of water at different driving pressure was investigated in detail. As shown in Table 1, when the applied pressure increases from 40 to 80 mbar, the flow speed and flow rate of fluid both increase. But the 7



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flowing distance in outlet 2 (D2) is zero when the applied pressure increases to 50 mbar, then D2 increases with the increasement of applied pressure (supporting information, Figure S3). In all experiments, the distance in outlet 1 (D1) is fixed at 5 mm (The microscopy images are just small part of outlet 1). It should be noted that, once water flowed through outlet 1, the water flow in outlet 2 would not change any more, even water accumulated to a big droplet, which demonstrating the stability of the gas-water interface in outlet 2. When the applied pressure is larger than 50 mbar, water shows anisotropic wetting flow (do not include unidirectional flow in this paper) in the microchannel. Therefore, for flow motion of water in such a micro-device, there should be a threshold pressure (failure pressure), and water flows unidirectionally when the applied pressure is smaller than the threshold pressure, while it flows anisotropically when it surpasses the thresold value. So for afore-mentioned microchannel in above experiment, the failure pressure is 50 mbar. Moreover, microchannels built upon different patterned surfaces may possess different failure pressure, and this pressure represents the controlling performance of patterned surfaces for fluid in microchannel. So for microchannels built upon patterned surfaces, failure pressure is a judging criteria for their controlling property. 3.3 Factors Influencing the Unidirectional Flow of Water in the Microchannel From the above experiment, we know that anisotropic wetting of the morphology patterned surfaces induces the unidirectional flow behavior of water in microchannel. Flow behavior of fluid in microchannel is determined by the interaction between fluid and surrounding walls, especially for our device, the underlying patterned surface is predominant. So factors that influence the interaction between fluid and patterned surfaces may affect flow behavior of fluid in microchannels, including wetting anisotropy of the patterned surfaces and dimensions of the channels. For morphology patterned surface of stripe structure, wetting anisotropy of surfaces vary from period of pattern and depth of the structure.30 Besides, for a fixed stripe structure, different 8


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chemical modification may also influence the flow behavior of fluid. In the following section, we studied all of the above mentioned influencing factors. 3.3.1 Morphology of the Si stripes According to the earlier reported literature, wetting anisotropy of periodic groove (or stripe) structured surfaces vary from period of pattern and depth of the structure.30 In addition, for a given material, changes in the surface roughness also influence the wetting property of the materials.28,39,40 Thus, the influences of period of patterns, depth of stripe structure, roughness of the surfaces on flow behavior of water in microchannels are investigated. Period of the stripes. In order to investigate how the period of patterns affects unidirectional flow of water in the microchannel, five patterned Si surfaces (600 nm in depth) with different period were prepared. As illustrated in Table 2, when period increases from 20 to 60 µm, failure pressure increases from 50.0 to 54.1 mbar. At the same time, WCAs on the five surfaces were measured. As we see from the table that the wetting anisotropy ∆CA (∆CA = CA∥- CA⊥) increases with the increasement of period of patterns. Based on the above experimental results, we consider the degree of wetting anisotropy affects the value of failure pressure, in other words, it determines the controlling capability of patterned surfaces for fluid in microchannels. Therefore, a big period is better for controlling flow behavior of water in our micro-device. Depth of the patterns. Here, we fabricated Si stripes (period of patterns is 20 µm) of different depth to study the influence of depth of structure on fluid motion in microchannels. The relationship between depth of Si stripes, wetting anisotropy, and failure pressure are shown in Table 3. Wetting anisotropy increases from 20.5o to 25.4o when the depth of the stripes increases from 410 to 1750 nm. The increasement in wetting anisotropy causes change in failure pressure from 50.0 to 54.2 mbar. So a patterned surface with deep stripes can tolerate a higher applied pressure, under which condition fluid cannot flow into outlet 2.

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Roughness of the Si stripes. Si stripes were further etched to fabricate Si stripes with different roughness. In the RIE process, we used Au NPs as etching masks. Au NPs with diameter about 16 nm were synthesized using a general seeded growth route (TEM microscopy image is shown in Figure S4a).36 The etching duration of Si was 10, 20, 30，and 45 s, respectively. After RIE process, Au NPs masks were removed by gold etchant, and Si stripes with different nanoscale roughness were produced. The AFM images of the prepared nanoscale structures are shown in Fig. S3b-e. The surface roughness of the surfaces increases from 1.670 to 12.40 nm when etching duration increases from 10 to 45 s (Table S1). After modification of PFS, WCAs in parallel and perpendicular directions were measured and wetting anisotropies were calculated. As shown in Table 4, Si stripes etched different time show different wetting anisotropy. Flow motions of water on these surfaces under different applied pressure were studied. The ∆CA decreases with the increasement of the surface roughness of the stripes, followed by the weakening of anisotropic wetting flow. When the surface roughness of the stripes increases from 0 to 4.145 nm, the failure pressure decreased from 50 to 46 mbar. Further increase in the roughness leads to a further decrease in the ∆CA. When ∆CA decreased to some extent, water flow in the microchannel upon this patterned surface will not show unidirectional flow, even driven by a relatively low driving pressure (Note: applied pressures lower than 45 mbar were not examined, because at such pressure, water would not flow in the microchannels due to the hydrophobic property of PDMS walls and Si stripes, especially existence of the stripe patterns provides a big energy barrier, and the water flow in inlet channel is very slow). The above experiments demonstrate that the morphology of stripe-patterned Si surfaces influences the flow behavior of water in microchannels. Changes in the above parameters lead change of wetting anisotropy of the patterned surfaces, resulting in different controlling capacity for fluid in microchannels. 3.3.2 Modification of the Si stripes 10


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Besides the morphology of Si stripes, the modified material also influences controlling capacity of patterned surfaces for fluid in microchannel. To demonstrate this, we modified the Si stripes with APTES monolayer. The modification of stripes with APTES is similar to PFS monolayer. The WCA on APTES modified flat silicon wafer is 57 o, while it is 113 o on PFS modified one, thus the surface energy of APTES is larger than PFS monolayer. As shown in Figure S5, the flow speed of water in two outlets was different, and water showed anisotropic flow in the microchannel, but water continued to flow in outlet 2 even it flowed through outlet 1. This is different from water flow in microchannel upon PFS modified Si stripes. The reason is the energy barrier of APTES modified Si stripes in perpendicular is low. Water shows anisotropic wetting in parallel and perpendicular directions, but essentially, the surface is hydrophilic in both directions. Therefore, a conclusion can be made that the performance of morphology patterned surfaces modified with low surface energy materials is well for controlling fluid flow in microchannel, the reason is not only water shows large wetting anisotropy on its surface, but large energy barrier in perpendicular direction in microchannels. Inspired from the above experiment, we believe that modification of stimuli-responsive materials onto patterned surfaces, such as photo- or thermo-responsive materials, will be helpful to realize smart control of fluid motion in microchannels, which should show a broad range of applications in future microfluidic systems.11 3.3.3 Dimension of the microchannels High surface-to-volume ratio is one of the main characters of microfluidics, thus changing in the size of microchannels will influence the controlling capability of patterned surface for water flow in the channels. To prove this, microchannels with same width but different height, and same height with different width were fabricated, and flow motion of water in these channels was studied. As shown in Table 5, when the height of the channel increases, the corresponding failure pressure is decreased, and when it increases to some extent, for example 63.5 µm in our experiment, water does not show unidirectional flow any more (supporting 11



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information, Figure S6). When height of channel increases, the proportion of pattern for the whole channel walls decreases, and energy barrier in the beginning of outlet 2 also decreases. This results in the weakening of unidirectional flow of water, namely, the controlling capability of the pattern. When the width of channel increases from 100 to 300 µm, failure pressure decreases from 54.5 to 45.0 mbar, because energy barrier in perpendicular direction is larger in a narrow outlet channel. So the dimension of microchannels also affects tailoring capacity of patterned surfaces for fluid in channels. Patterned surface shows a better controlling function for fluid in narrow and low channels. 3.4 Performance of the micro-device The unidirectional flow of water in the microchannel upon morphology patterned surfaces is originated from the anisotropic wettability of water on the patterned surfaces. It is well known that the surface tension of water is relative large, so it exhibits large wetting anisotropy on the morphology patterns, but fluids with low surface tension have low wetting anisotropy when they are dropped onto the patterns.29 Considering this, we suspect that flow motion of fluids with different surface tension in a same microchannel upon patterned surface are different, or the controlling capabilities of same pattern for fluids with different surface tension are different. To prove our assumption, we studied flow behavior of fluids with different surface tension in the microchannel at a same driving pressure. First, solutions of different mass fraction of ethanol were prepared and their surface tension values were measured using pendant drop method (Dataphysics OCA20). As shown in Table S2, surface tension of fluids decreases with the increase of the fraction of ethanol. Under a same driving pressure of 50 mbar, the flowing distance of fluids in outlet 2 is different (Figure 3). When surface tension changed from 71.93 to 56.47 mN/m, the fluids just flowed a small distance in outlet 2. The flow distance becomes apparently larger when surface tension continues to decrease. We ascribe this phenomenon to that, when surface tension of fluids decreases, it becomes easier for fluids to wet in the perpendicular direction, thus the energy barrier that needs to overcome 12


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is low. Although fluid with low surface energy flows for a long distance in outlet 2, the position of gas-fluid interface would not change once it flows through outlet 1. This is similar to fluid with high surface energy. Besides water and solution of different fraction of ethanol, we also studied flow bahavior of other liquids in the microchannel. We found that liquids with relatively low surface tension, such as ethanol, liquid paraffin, and ethylene glycol, flowed through from the both outlets under an applied pressure of 50 mbar, but liquids with relatively high surface tension, such as aqueous solution of sodium chloride, bovine serum albumin, and water suspension of polystyrene microspheres show unidirectional flow in our microchannel. Thus we can conclude that morphology patterned surfaces prefer to manipulate fluids with higher surface tension. For device designing, performance of the device is quite important, such as stability, and durability, etc. For our micro-device, stability refers to the unidirectional flow of fluid in the channels. We tested water flow in the microchannel for a period of time, and the gas-water interface in the beginning of outlet 2 was tracked in real-time. As shown in Figure S7, water showed unidirectional flow character at a driving pressure of 45 mbar, and unidirectional flow of water remained unchanged for 30 min. The gas-water interface did not move when water flowed through outlet 1 and accumulated to a big droplet and dropped down from the glass slide under the micro-device. Even the applied pressure gradually increased from 45 to 70 mbar, the gas-water interface also remained its initial position, indicating once the interface was formed it was quite stable. Apart from this, the durability of the device is pretty good. Due to the low surface energy of PFS layer, fluids hardly stick to the patterned surfaces, and cleaning of surfaces is performed simply by rinsing surfaces with ethanol and drying with nitrogen gas stream. The surface is reusable and failure pressure of water in the microchannel does not change even the surface is used over a hundred time. Compared to chemically patterned surfaces, the stability of physically patterned surface is better (Figure S8). It should be noted that, suffering from the reversible bonding of patterned surface and PDMS 13



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microchannel, the leakage pressure of our device is relatively low. As shown in Figure S7, when applied pressure was 200 mbar, fluid leakage occurred from the two plates; therefore, the bonding property of our device is poor compared to chemically bonded ones. But at a pressure of 200 mbar, the flow speed of fluid in the microchannel is very high, in most situations, flow speed in microchannels is much lower than this value. Considering the stability and durability of our micro-device, and benefit from their simple fabrication processes, our device is more suitable for practical applications compared to previously developed asymmetric structures. 3.5 Applications of the unidirectional flow 3.5.1 “Valve” function: controlling flow path by adjusting the applied pressure Water presented unidirectional flow in microchannels upon morphology patterned surfaces, indicating the fluid motion in outlet 2 was largely suppressed, thus the morphology patterned surface could be used as a passive micro-valve in the microchannel.21 As water shows different flowing behavior at different driving pressure, the close-open-close procedure of the micro-valve can be changed by adjusting the applied pressure (Figure 4). As shown in Fig. 4a-b, when the driving pressure was low (45 mbar), water showed unidirectional flow in microchannel, so outlet 1 was open and outlet 2 was “closed”. When the pressure changed to 100 mbar, water broke the initial gas-water interface and it flowed through outlet 2, then outlet 2 channel opened (Figure 4c-d). To close the two outlets, water in the channel was ejected by N2 gas. When the driving pressure of gas was 80 mbar, N2 pushed the water in outlet 1 away, but water in outlet 2 remained unchanged (Figure 4e-f). This is because water flow in parallel direction is easy, but in perpendicular direction, there exist an energy barrier to overcome. When the driving pressure of gas changed to 150 mbar, water in outlet 2 was ejected, and both outlets were “closed” (Figure 4g-h). To open the outlet 1 again, water was injected at a pressure of 45 mbar (Figure 4i-l). Thus, morphology patterned surfaces serves as

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a micro-valve and close-open-close procedure of micro-valve can be controlled by the driving pressure of fluid and gas. 3.5.2 Gas-liquid separation in microchannel based on the unidirectional flow Droplet-based microfluidics is one important part of microfluidics, which has shown to be compatible with many chemical and biological reagents and becomes a potent high throughput platform for biomedical research and applications.41,42 Droplet-based systems focus on creating discrete volumes with the use of immiscible phases. So far, various droplet operation methods have been developed, including droplet generation, fission, fusion, mixing, and sorting. But separations of the immiscible phases have attracted limited attention compared to above mentioned operations, although more research is directed towards this area. Actually, gas-liquid and liquid-liquid multiphase separations are indispensible when requiring separation of different phases from each other for further analysis or processing in microfluidics.43 So far, two immiscible phases in microchannels are mainly separated by using gravity,44 capillary,45 and Laplace forces46-48. Here, we develop one alternative method for gas-liquid separation in microfluidics, which is based on unidirectional flow of water in microchannel upon morphology patterned anisotropic wetting surfaces. Some air plugs and water plugs were formed in the PTFE pipe, and they were injected to microchannel at a pressure of 52 mbar. As shown in Figure 5a, when water arrived at the Y-junction, it was directed to outlet 1 due to anisotropic wetting of patterned surface. When air plugs reached the junction, it pushed water to a small distance until it broke the gas-water interface in the beginning of outlet 2 (Figure 5b-c). Once the gas-water interface was broken, air plugs entered and then flowed away from outlet 2 (Figure 5d), because the hydrodynamic resistance for the flow of air plug in outlet 1 was significantly higher than that in outlet 2. At the same time, the position of water in outlet 1 remained unchanged until the next water slugs arrived to the junction (Figure 5e-g). It should be noted that although a small part of air exist in outlet 1, it was pushed to outlet 2 when water arrives the junction (Figure 5h), as a result, water and air 15



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flowed into outlet 1 and 2, respectively. The air-water separation process continued when the next air plugs and water slugs were injected to the channel (Figure 5i-l). The corresponding video that records entire gas-water separation process is shown in the Video S2 (supporting information) and a schematic illustration of gas-water separation process is shown in Figure S9 for understanding the separation mechanism. But in microchannel built upon chemically patterned surface, gas-water separation could not be achieved because of the anisotropic wetting flow behavior of water in the channel (Figure S10). For our microfluidic device, two conditions must be satisfied for gas-liquid separation: first, when water reaches the junction for the first time, flow distance of water in outlet 2 is short; second, the gas-water interface must be broken when the air plugs arrives the junction. Besides, we found that there is little air bubble exists in the liquid phase, and no water slugs found in the air, thus we believe that the separation efficiency of gas and water is nearly 100% in our micro-device. The above-mentioned results demonstrate the morphology patterned anisotropic wetting surface provide an alternative method for gas-liquid separation in microfluidics. 4. Conclusion In this paper, we present a simple but useful strategy to direct flow motion of fluid within a microchannel using anisotropic wetting surfaces of morphology patterned stripes. The anisotropic flow behavior of fluid in channels is originated from anisotropic wetting property of the patterned surfaces and flow behavior is influenced by the value of driving pressure, period of pattern, depth of stripe structure, and size of the microchannels. In addition, fluids with different surface tension show different flowing anisotropy in our micro-device. The morphology patterned surfaces could be used as a micro-valve and gas-water separation in microchannel was realized based on the unidirectional flow of water. Thus, morphology patterned surfaces are an alternative tool for fluid control in microfluidics, and benefiting from their simple fabrication process, they will be more suitable for practical applications compared to previously developed asymmetric structures. Additionally, smart control of fluid 16


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motion may be realized by introducing patterned surfaces modified by stimuli-responsive materials, such as photo- or thermo-responsive materials, within microchannels, which would meet future demand of microfluidic chips. Supporting Information Detailed flow behavior of water in Y-shaped microchannel on PFS modified patterned surface with Si stripes; Mechanism of the occurance of energy barrier in microchannels built on morphology and chemically patterned surface; Optical microscopy images of water flowing distance in Y-shaped microchannel on PFS modified Si stripes at different driving pressure; TEM image of the synthesized Au NPs and AFM images of the nanoscale structures of different roughness; Relationship between etching duration, height of the nano structures and roughness of the stripes; Surface tension values of solution of different mass fraction of ethanol, and their detailed flowing parameters in microchannel upon Si stripes; Flowing process of water in microchannel built on the APTES modified Si stripes and microchannel with large height; Stability of gas-water interface in the beginning of outlet 2; Comparison of the stability between the physical and chemical patterns; A schematic illustration of gas-water separation process; Failure of gas-water separation in microchannel built upon chemcially patterned surface of SiOH-PFS stripes. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Basic Research Program of China (2012CB933800), the Natural Science Foundation of China (Grant no. 21222406, 21474037), the Program for New Century Excellent Talents in University, Doctoral Fund of Ministry of Education of China (20130061110019), and Science and Technology Development Program of Jilin Province. References 17



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(1) Whitesides, G. M. The Origins and the Future of Microfluidics. Nature 2006, 442, 368-373. (2) Medina-Sánchez, M.; Miserereab, S.; MerkoçI, A. Nanomaterials and Lab-on-a-Chip Technologies. Lab Chip 2012, 12, 1932-1943. (3) Auroux, P. A.; Iossifidis, D.; Reyes, D. R.; Manz, A. Micro Total Analysis Systems. 2. Analytical Standard Operations and Applications. Anal. Chem. 2002, 74, 2637-2652. (4) Reyes, D. R.; Iossifidis, D.; Auroux, P. A.; Manz, A. Micro Total Analysis Systems. 1. Introduction, Theory, and Technology. Anal. Chem. 2002, 74, 623-2636. (5) Nagrath, S.; Sequist, L. V.; Maheswaran, S.; Bell, D. W.; Irimia, D.; Ulkus, L.; Smith, M. R.; Kwak, E. L.; Digumarthy, S.; Muzikansky, A.; Ryan, P.; Balis, U. J.; Tompkins, R. G.; Haber, D. A.; Toner, M. Isolation of Rare Circulating Tumour Cells in Cancer Patients by Microchip Technology. Nature 2007, 450, 1235-1239. (6) Zhang, C.; Xing, D. Single-Molecule DNA Amplification and Analysis Using Microfluidics. Chem. Rev. 2010, 110, 4910-4947. (7) Oh K. W.; Ahn C. H. A Review of Microvalves. J. Micromech. Microeng. 2006, 16, R13-R39. (8) Yu Q.; Bauer J. M.; Moore J. S. Responsive Biomimetic Hydrogel Valve for Microfluidics. Appl. Phys. Lett. 2001, 78, 2589-2591. (9) Zhao, B.; Moore, J. S.; Beebe, D. J. Surface-Directed Liquid Flow Inside Microchannels. Science 2001, 291, 1023-1026. (10) Chen C.; Xu P.; Li X. Regioselective Patterning of Multiple SAMs and Applications in Surface-Guided Smart Microfluidics, ACS Appl. Mater. Interfaces 2014, 6, 21961-21969. (11) Wang, T.; Chen, H.; Liu, K.; Wang, S.; Xue, P.; Yu, Y.; Ge, P.; Zhang, J.; Yang, B. Janus Si Micropillar Arrays with Thermal-Responsive Anisotropic Wettability for Manipulation of Microfluid Motions. ACS Appl. Mater. Interfaces 2015, 7, 376-382. (12) Tsougeni, K.; Papageorgiou, D.; Tserepi, A.; Gogolides, E. ‘‘Smart’’ Polymeric 18


Page 18 of 27

Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Microfluidics Fabricated by Plasma Processing: Controlled Wetting, Capillary Filling and Hydrophobic Valving. Lab Chip 2010, 10, 462-469. (13) Oliveira, N. M.; Neto, A. I.; Song, W.; Mano, J. F. Two-Dimensional Open Microfluidic Devices by Tuning the Wettability on Patterned Superhydrophobic Polymeric Surface. Appl. Phys. Express 2010, 3, 085205. (14) Bai, Z.; He, Q.; Huang, S., Hu, X.; Chen, H. Preparation of Hybrid Soda-Lime/Quartz Glass Chips with Wettability-Patterned Channels for Manipulation of Flow Profiles in Droplet-Based Analytical Systems. Anal. Chim. Acta 2013, 767, 97-103. (15) Logtenberg, H.; Lopez-Martinez, M. J.; Feringa, B. L.; Browne, W. R.; Verpoorte, E. Multiple Flow Profiles for Two-Phase Flow in Single Microfluidic Channels through Site-Selective Channel Coating. Lab Chip 2011, 11, 2030-2034. (16) Xiao, H.; Liang, D.; Liu, G.; Guo, M.; Xing, W.; Cheng, J. Initial Study of Two-Phase Laminar Flow Extraction Chip for Sample Preparation for Gas Chromatography. Lab Chip 2006, 6, 1067-1072. (17) Kim, T.-i.; Suh, K. Y. Unidirectional Wetting and Spreading on Stooped Polymer Nanohairs. Soft Matter 2009, 5, 4131-4135. (18) Kwak, M. K.; Jeong, H.-E.; Kim, T.-i.; Yoon, H.; Suh, K. Y. Bio-Inspired Slanted Polymer Nanohairs for Anisotropic Wetting and Directional Dry Adhesion. Soft Matter 2010, 6, 1849-1857. (19) Yildirim, A.; Yunusa, M.; Ozturk, F. E.; Kanik, M.; Bayindir, M. Surface Textured Polymer Fibers for Microfluidics. Adv. Funct. Mater. 2014, 24, 4569-4576. (20) Bae, W.-G.; Kim, S. M.; Choi, S.-J.; Oh, S. G.; Yoon, H.; Char, K.; Suh, K. Y. In Situ Realization of Asymmetric Ratchet Structures within Microchannels by Directionally Guided Light Transmission and Their Directional Flow Behavior. Adv. Mater. 2014, 26, 2665-2670. (21) Wang, T.; Chen, H.; Liu, K.; Li, Y.; Xue, P.; Yu, Y.; Wang, S.; Zhang, J.; Kumacheva, E.; 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Yang, B. Anisotropic Janus Si Nanopillar Arrays as a Microfluidic One-Way Valve for Gas-Liquid Separation. Nanoscale 2014, 6, 3846-3853. (22) Chu, K.-H.; Xiao, R.; Wang, E. N. Uni-Directional Liquid Spreading On Asymmetric Nanostructured Surfaces. Nat. Mater. 2010, 9, 413-417. (23) Hancock M. J.; Sekeroglu K.; Demirel M.C. Bioinspired Directional Surfaces for Adhesion, Wetting, and Transport Adv. Funct. Mater. 2012, 22, 2223-2234. (24) Xia, D.; Johnson, L. M.; López, G. P. Anisotropic Wetting Surfaces with One-Dimesional and Directional Structures: Fabrication Approaches, Wetting Properties and Potential Applications. Adv. Mater. 2012, 24, 1287-1302. (25) Zhang, P.; Liu, H.; Meng, J.; Yang, G.; Liu, X.; Wang, S.; Jiang, L. Grooved Organogel Surfaces towards Anisotropic Sliding of Water Droplets. Adv. Mater. 2014, 26, 3131-3135. (26) Wang T.; Li X.; Zhang J.; Wang X.; Zhang X.; Zhang X.; Zhu D.; Hao Y.; Ren Z.; Yang B. Elliptical Silicon Arrays with Anisotropic Optical and Wetting Properties. Langmuir 2010, 26, 13715-13721. (27) Zhang J.; Yang B. Patterning Colloidal Crystals and Nanostructure Arrays by Soft Lithography. Adv. Funct. Mater. 2010, 20, 3411-3424. (28) Zhang J.; Li Y.; Zhang X.; Yang B. Colloidal Self-Assembly Meets Nanofabrication: From Two Dimensional Colloidal Crystals to Nanostructure Arrays. Adv. Mater. 2010, 22, 4249-4269. (29) Morita, M.; Koga, T.; Otsuka, H.; Takahara, A. Macroscopic-Wetting Anisotropy on the Line-Patterned Surface of Fluoroalkylsilane Monolayers. Langmuir 2005, 21, 911-918. (30) Zhao, Y.; Lu, Q.; Li, M.; Li, X. Anisotropic Wetting Characteristics on Submicrometer-Scale Periodic Grooved Surface. Langmuir 2007, 23, 6212-6217. (31) Chung J. Y.; Youngblood J. P.; Stafford C. M. Anisotropic Wetting on Tunable Micro-Wrinkled Surfaces. Soft Matter 2007, 3, 1163-1169. (32) Xia D.; Brueck S. R. J. Strongly Anisotropic Wetting on One-Dimensional Nanopatterned 20


Page 20 of 27

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Surfaces. Nano Lett. 2008, 8, 2819-2824. (33) Xia D.; He X.; Jiang Y,; Lopez G. P., Brueck S. R. J. Tailoring Anisotropic Wetting Properties on Submicrometer-Scale Periodic Grooved Surfaces. Langmuir 2010, 26, 2700-2706. (34) Wang S.; Wang T.; Ge P.; Xue P.; Ye S.; Chen H.; Li Z.; Zhang J.; Yang B. Controlling Flow Behavior of Water in Microfluidics with a Chemically Patterned Anisotropic Wetting Surface. Langmuir 2015, 31, 4032-4039. (35) Zhu, D.; Li, X.; Zhang, G.; Zhang, X.; Zhang, X.; Wang, T.; Yang, B. Mimicking the Rice Leaf

From Ordered Binary Structures to Anisotropic Wettability. Langmuir 2010, 26,

14276-14283. (36) Bastús N. G.; Comenge J.; Puntes V. Kinetically Controlled Seeded Growth Synthesis of Citrate-Stabilized Gold Nanoparticles of up to 200 nm: Size Focusing versus Ostwald Ripening. Langmuir 2011, 27, 11098-11105. (37) Ruan W.; Zhou T.; Hui G.; Wang Y.; Chong X.; Wang X.; Song W.; Han X.; Zhao B. Particle Lithography-Based Patterning of Polyelectrolyte Template Films and Their Application in Fabrication of Gold/Silver Nanoparticle Assembly. J. Colloid Interface Sci. 2014, 432, 65-69. (38) Xu, B.-Y.; Yan, X.-N.; Zhang, J.-D.; Xu, J.-J.; Chen, H.-Y. Glass Etching to Bridge Micro- and Nanofluidics. Lab Chip 2012, 12, 381-386. (39) Wang J.; Zheng Y.; Nie F.-Q.; Zhai J.; Jiang L. Air Bubble Bursting Effect of Lotus Leaf. Langmuir 2009, 25, 14129-14134. (40) Wang S.; Liu K.; Yao X.; Jiang L. Bioinspired Surfaces with Superwettability: New Insight on Theory, Design, and Applications. Chem. Rev. 2015, 115, 8230-8293. (41) Teh S.-Y.; Lin R.; Hung L.-H.; Lee A. P. Droplet Microfluidics. Lab Chip, 2008, 8, 198-220. (42) Shui L.; Eijkel J. C. T.; Berg A. v. d. Multiphase Flow in Microfluidic Systems – Control 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and Applications of Droplets and Interfaces. Adv. Colloid Interface Sci. 2007, 133, 35-49. (43) Lam K. F.; Sorensen E.; Gavriilidis A. Review on Gas–Liquid Separations in Microchannel Devices. Chem. Eng. Res. Des. 2013, 91, 1941-1953. (44) Irandoust S.; Andersson B. Liquid Film in Taylor Flow through a Capillary. Ind. Eng. Chem. Res. 1989, 28, 1684-1688. (45) Günther A.; Jhunjhunwala M.; Thalmann M.; Schmidt M.A.; Jensen K.F. Micromixing of Miscible Liquids in Segmented Gas-Liquid Flow. Langmuir 2005, 21, 1547-1555. (46) Aota A.; Hibara A.; Kitamori T. Pressure Balance at the Liquid-Liquid Interface of Micro Countercurrent Flows in Microchips. Anal. Chem. 2007, 79, 3919-3924. (47) Sahoo H. R.; Kralj J. G.; Jensen K. F. Multistep Continuous-Flow Microchemical Synthesis Involving Multiple Reactions and Separations. Angew. Chem. Int. Ed. 2007, 46, 5704 -5708. (48) Aota A.; Mawatari K.; Takahashi S.; Matsumoto T.; Kanda K.; Anraku R.; Hibara A.; Tokeshi M.; Kitamori T. Phase Separation of Gas–Liquid and Liquid–Liquid Microflows in Microchips. Microchim Acta 2009, 164, 249-255.

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Figure 1. (a) Optical microscopy image of patterned surface with photoresist stripes. The scale bar is 50 µm. (b) The SEM image of patterned surface with Si stripes. Contact angle of water on patterned surface with Si stripes (c) viewing from perpendicular direction; (d) viewing from parallel direction.

Figure 2. (a) The schematic illustration of the fabrication of microchannel upon morphology patterned surfaces. (b) Optical microscopy image of the Y-shaped microchannel (top = 146 µm, bottom = 200 µm, height = 25 µm). The black arrow represents the injecting direction of water. The flow behavior of water in Y-shaped microchannel bonding (c) PFS modified patterned surface with Si stripes, (d) PFS modified flat Si surface and (e) patterned surface with photoresist stripes. The black lines represent the water position in two outlet channels and the arrows represent the water running distance in two outlets. The scale bar is 1 mm and driving pressure is 45 mbar. 23



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Table 1. Detailed flowing parameters of water in Y-shaped microchannel on PFS modified Si stripes driven by different applied pressure. Applied pressure

Flow speed

Flow rate

D2

[mbar]

[mm s-1]

[μL min-1]

[mm]

D2/D1

40

0.09

0.02

0.00

0.000

45

0.75

0.20

0.00

0.000

50

3.09

0.80

0.00

0.000

55

7.01

1.82

0.43

0.086

60

13.88

3.60

0.74

0.148

70

21.78

5.65

0.78

0.156

80

35.55

9.23

2.53

0.506

Table 2. Water contact angle values on Si stripes of different period and the corresponding failure pressure in microchannels on these stripes. All the stripes are 600 nm in depth. Period

CA∥

CA⊥

ΔCA

Failure pressure

[μm]

[°]

[°]

[°]

[mbar]

20

141.0±1.0

121.2±1.1

19.8

50.0±0.5

30

139.4±0.4

117.3±1.5

22.1

51.3±0.7

40

136.6±2.2

111.2±2.2

25.4

52.1±0.4

50

140.4±3.1

112.5±1.3

27.9

53.0±0.5

60

140.3±1.9

110.4±1.4

29.9

54.1±0.6

Table 3. Water contact angle values on Si stripes of different depth and the corresponding failure pressure in microchannels on these stripes. All the stripes are 20 µm in period. Depth

CA∥

CA⊥

ΔCA

Failure pressure

[nm]

[°]

[°]

[°]

[mbar]

410

134.8±1.7

114.3±1.9

20.5

50.0±0.5

600

141.0±1.0

121.2±1.1

19.8

50.0±0.5

1130

139.5±0.4

117.8±2.4

21.7

51.5±0.8

1370

139.8±0.3

118.5±2.0

22.3

52.2±0.7

1750

138.5±0.5

113.1±3.9

25.4

54.2±1.0

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Table 4. Water contact angle values on Si stripes with different roughness and detailed flowing parameters of water in microchannels on these stripes driven by different applied pressure. All the stripes are 20 µm in period, and 600 nm in depth. Roughness

CA∥

CA⊥

ΔCA

Applied pressure

D2

[nm]

[°]

[°]

[°]

[mbar]

[mm]

0

141.0±1.0

121.2±1.1

19.8

50

0.00

0.000

Unidirectional

1.670

138.2±1.6

117.3±1.2

19.9

47

0.00

0.000

Unidirectional

4.145

137.3±0.4

119.3±1.8

18.0

46

0.00

0.000

Unidirectional

8.928

137.8±0.6

123.5±1.8

14.3

45

0.18±0.03

0.036

Anisotropic

12.400

139.3±0.3

134.1±1.2

5.2

45

0.37±0.05

0.074

Anisotropic

D2/D1

Flowing characteristic

Table 5. Failure pressure in microchannels with different dimension. The underlying stripes are 20 µm in period, and 600 nm in depth. Channel No.

Bottom width

Top width

Height

Failure pressure

[μm]

[μm]

[μm]

[mbar]

1

200

146

25

50.0±0.5

2

200

100

48

35.4±0.6

3

200

75

63.5

~30

4

100

47

25

54.5±0.5

5

300

250

25

45.0±1.0

Figure 3. Optical microscopy images of flowing distance of fluids with different surface tension in the Y-shaped microchannel on the PFS modified Si stripes. The surface tension of the fluid in (a-f) is 71.93, 64.61, 56.47, 49.49, 43.22, 40.04 mN m-1, respectively. The applied pressure is 50mbar and scale bar is 1 mm. (g) The relationship between surface tension of the fluid and flow distance in outlet 2.

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Figure 4. Close-open-close process of micro-valve adjusted by changes of applied pressure. Microscopy images of water (a-b) at a driving pressure of 45 mbar, (c-d) when the pressure increases from 45 to 100 mbar. Water ejected by N2 gas (e-f) at pressure of 80 mbar, (g-h) when the pressure increases from 80 to 150 mbar. (i-l) Water was injected at a pressure of 45 mbar, again. The black (blue) lines represent the water (gas) position and the arrows represent the water (gas) running distance in two outlet channels. The scale bar is 1mm.

Figure 5. Gas-water separation in microchannel upon morphology patterned anisotropic wetting surface. (a) Water plug arrived at the Y-junction and flowed through outlet 1. (b-d) The air slug reached the junction and flowed through outlet 2. (e-h) The next water plug arrived and also flowed through outlet 1. (i-l) The above gas-water separation was repeated in the following process. The black arrows represent the flow direction of water and the blue arrows represent the flow direction of air. The scale bar is 1 mm and driving pressure is 52 mbar. 26


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The table of contents entry: Anisotropic wetting surfaces of morphology patterned stripes are integrated with microfluidic channel. Water shows unidirectional flow characteristic in the microchannel due to the anisotropic wetting property of the patterned surfaces. Based on the unidirectional flow of water, separation of segmented gas-water flow is achieved in the microchannel, which provides another strategy for separation of immiscible phases for microfluidics. Keyword: Microfluidics, Surface patterning, Anisotropic wetting, Fluid control, Gas-liquid separation Morphology Patterned Anisotropic Wetting Surface for Fluid Control and Gas-Liquid Separation in Microfluidics

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Morphology-Patterned Anisotropic Wetting Surface for Fluid Control

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